The present invention relates to an apparatus for producing an extruded hollow plastic section in accordance with the preamble of claim 1. Such an apparatus for producing an extruded plastic tube is known, for example, from NL 7612518 and NL 1001259.
In the apparatus which is known from NL 7612518, the first sealing plug is disposed immediately downstream of the inner mandrel of the die, between the die and the external calibration device, while the second sealing plug is situated downstream of the external cooling device. The two sealing plugs together with the extruded tube delimit a cooling water compartment in the inner chamber of the tube. A feed line is provided for filling cooling water into said compartment. Furthermore, a discharge line and suitable pump means are provided, so that cooling water can be circulated through the cooling water compartment, with the result that the extruded tube is cooled internally.
The closure obtained by the first sealing plug has to be very reliably liquid-tight, in order to prevent the hot inner mandrel of the die from coming into contact with the cooling water. In practice, such contact leads to considerable disruption or jamming of the extrusion process and may even damage the inner mandrel.
It is known that if an extruded plastic tube is cooled not only externally but also internally it is possible to achieve a considerably higher cooling capacity than if cooling takes place only externally, this increased cooling capacity theoretically being greater by a factor of four. An important practical advantage of this is that the length of the cooling assembly of the extrusion apparatus can be made considerably shorter. It is also possible, if the length of the cooling assembly is kept the same, to increase the output of the extruder.
Another generally accepted advantage of the combination of external and internal cooling of a plastic tube, in particular thick-walled pipe, by comparison with exclusively external cooling, is that the tube coming out of the extruder then quickly acquires a cooled, and hence relatively strong layer, both on the internal circumference and on the external circumference. The two cooled layers enclose the warm, soft plastic material between them, thus preventing, or at least counteracting, the possibility of this soft plastic material sagging downwards, resulting in an undesired shape of the tube.
Despite the abovementioned known advantages internal cooling, in particular in combination with extrenal cooling, is little used in the industry for the production of extruded hollow plastic sections. This is because the use of internal cooling leads to a number of problems which have hitherto not been solved satisfactorily. One of these problems is that the known devices and methods for internal cooling do not allow a uniform cooling of the extruded section, which has the undesirable effect that the inside of the extruded section showns an irregular surface and the wall thickness also exhibits undesirable deviations. Another problem relates to the constriction of the plastic section coming out of the extruder. Particularly when extruding polyolefin tubes, the extruded mass of plastic material comes out of the die at a speed which is lower than the speed of the tube at the location of the drawing or hauloff machine. As a result, the wall thickness of the tube decreases and also the diameter of the tube becomes smaller. Since a cold and strong layer is rapidly formed on the inside of the tube if internal cooling is used, this layer will start to become creased as the diameter of the tube is reduced.
The object of the present invention is to provide an apparatus and a method with which the internal cooling process in the horizontal extrusion of hollow plastic sections can be improved and controlled better, enabling better results to be achieved.
The present invention provides an apparatus which defines a path for said extruded plastic section which is such that the compartment which is filled with liquid has a highest point at a predetermined location, and in that the apparatus further comprises vent means for discharging from the liquid filled compartment gas or vapor which collects at the highest point of the said compartment.
In an embodiment which is advantageous in practice, the two sealing plugs bounding said liquid filled compartment are rigidly fixed by suitable attachment means to said inner mandrel of the die, and the rigid assembly of the die and the attached sealing plugs is directed at an incline with respect to the horizontal.
In another embodiment the external calibration device is vertically offset from the die of the extruder, so that between them an inclined path is formed for the extruded tube and the highest point of the liquid filled compartment can be obtained. In this arrangement both the die and the external calibration device can be arranged horizontally as a slight bending of the still soft extruded section is allowable in many cases. It is also envisaged that the die is oriented horizontally and that the two sealing plugs bounding the liquid filled compartment are vertically offset from one another to obtain an inclined path for the extruded section.
The present invention is based on the insight that, particularly if water is used for internally cooling the extruded plastic section, air bubbles are released from the cooling water when the water heats up as a result of coming into contact with the extruded plastic section which comes out of the extruder at a high temperature. The direct contact between the plastic section and the cooling water may also result in the water beginning to boil in certain locations, thus producing steam. The air bubbles and the steam accumulate in the top of the inner chamber in the plastic section. At the location where air and steam are situated in the top of the inner chamber in the plastic section, the plastic section is not in direct contact with the cooling water. The cooling of the plastic section is then no longer uniform when viewed in the circumferential direction of the plastic section. However, there is a continuous feed of still relatively cold cooling water at the location along the inner wall of the plastic section where the steam bubbles and air bubbles are formed, so that by contrast the plastic section is cooled more strongly at that location.
The present invention provides for the extruded section to be supported and guided by the extrusion apparatus in such a manner that the liquid-filled compartment has a highest point at a predetermined location and that the apparatus has vent means arranged to remove the gas and air bubbles which collect at said location. The air bubbles and vapour bubbles rising in the liquid-filled compartment move automatically towards the highest point, where they are discharged through the vent means.
The measure according to the invention is also advantageous when combined with measuring the wall thickness of the extruded section. It is usual to measure the wall thickness of an extruded section continuously, in order thus to control the gap of the die so as to obtain a wall thickness and shape of the section which are as uniform as possible. It has been found that if internal cooling is used, with a sealing plug which lies inside the plastic section forming the separation between the inner mandrel of the die and a compartment which is filled with cooling medium, it is extremely important that the wall thickness of the section coming out of the die be controlled accurately, in order to ensure the sealing action of the sealing plug. This is the case in particular if the cooling medium is pressurized. If the extruded section, which at the location of the sealing plug situated directly downstream of the die is still very hot and therefore soft, is locally thinner, it will be easy for cooling medium to leak past the sealing plug at that location and come into contact with the die. This situation is highly disadvantageous.
It is usual to carry out the wall-thickness measurement using ultrasound, each interface between different materials, for example between the plastic material of the section and a liquid, providing an echo. Known ultrasonic wall-thickness measuring devices are designed to use the echoes received to determine the wall thickness and the cross-sectional shape of the extruded plastic section. According to a preferred embodiment of the invention, it is provided for a liquid-filled compartment in the plastic section to be present at the location of the ultrasonic wall-thickness measuring device, said compartment being formed by a sealing plug upstream and a sealing plug downstream of said measuring device, and for vent means to be provided for discharging from that compartment gas or vapour which accumulates at the highest point of the said compartment. The fact that according to the invention the entire compartment is filled with liquid avoids disruption caused by air or vapour which would otherwise be present. The present invention also provides for an ultrasonic sound emitted by the wall-thickness measuring device to pass at a location from outside through the wall of the plastic section, resulting in two echoes, and then to propagate through the liquid in the plastic section, after which it again passes through the wall of the plastic section at the diametrically opposite location, resulting in a further two echoes. The wall-thickness measuring device can use the four echoes to determine the wall thickness at two locations which are situated diametrically opposite one another.